Lithium manganate, method of producing the same, and lithium cell produced by the method

ABSTRACT

The present invention relates to a lithium manganate useful as an active material of positive electrodes for lithium batteries, and a process for producing the same, a positive electrode which uses the same as an active material of positive electrode, and a lithium battery. The lithium manganate of the present invention has a cubic particle form and contains voids in the particles, and therefore lithium batteries using it as an active material of positive electrodes provides a high initial discharge capacity of at least 95 mAh/g and, besides, are excellent in cycle characteristics.

TECHNICAL FIELD

The present invention relates to lithium manganate which is a compounduseful as an active material for positive electrode for lithium battery,a process for producing the same, a positive electrode for lithiumbattery using the same as an active material for positive electrode, anda lithium battery.

BACKGROUND ART

Lithium manganate is a compound represented by the formulaLi_(x)Mn_(y)O₄, and representative are spinel type LiMn₂O₄,Li_(4/3)Mn_(5/3)O₄ and the like. For obtaining such lithium manganates,a process of firing a mixture of a manganese compound and a lithiumcompound at a temperature of about 800° C. is employed.

DISCLOSURE OF INVENTION

Lithium manganate obtained by the above conventional process is apt tobecome a sintered body in which non-uniform sintering occurs betweenparticles because a mixture of a manganese compound and a lithiumcompound is fired at about 800° C. for the purpose of adjustment ofvalence of manganese or diminishment of by-products. Thus, there is aproblem that size of particles cannot be controlled. Moreover, since amixture of a manganese compound and a lithium compound is inferior inreactivity even when it is fired at high temperatures, uniformcomposition can hardly be obtained and there are many lattice defects.In order to avoid these problems, firing or mechanical grinding must berepeated many times.

Furthermore, lithium secondary batteries which use lithium manganateobtained by the above process as an active material for positiveelectrodes are not only low in initial charge and discharge capacity,but also show conspicuous reduction in capacity with repetition ofcharge and discharge. This is because crystals of lithium manganate arecollapsed at the time of charging and discharging, which is consideredto be caused by the presence of lattice defects and the low lithium ionconductivity.

For the solution of the above problems, there is proposed a processwhich comprises impregnating a porous manganese dioxide with lithiumacetate, lithium nitrate or lithium hydroxide and obtaining a product ofuniform composition at low temperatures (for example, “Electro-chemistry(Denki Kagaku)”, 63, 941 (1995)), but this process is still notsufficient.

The inventors have conducted intensive research in an attempt to obtainlithium manganate useful as active materials for positive electrodes oflithium batteries. As a result, it has been found that lithium secondarybatteries in which is used an active material for positive electrodeswhich comprises lithium manganate having a cubic particle form andcontaining voids in the particles are high in initial charge anddischarge capacity and excellent in cycle characteristics afterrepetition of charge and discharge. After additional investigations, thepresent invention has been accomplished. That is, the present inventionrelates to a lithium manganate which has a cubic particle form andprovides an initial discharge capacity of at least 95 mAh/g when it isused as an active material for positive electrodes of lithium batteries.Furthermore, the present invention relates to a process for theadvantageous production of the lithium manganate, and the first processis characterized by including a step of reacting a manganese compoundwith an alkali to obtain a manganese hydroxide, a step of oxidizing thehydroxide in an aqueous medium or a gaseous phase to obtain a manganeseoxide, a step of reacting the manganese oxide with a lithium compound inan aqueous medium to obtain a lithium manganate precursor, and a step offiring the precursor with heating to obtain lithium manganate. Thesecond process is characterized by including a step of reacting amanganese compound with an alkali to obtain a manganese hydroxide, astep of oxidizing the hydroxide in an aqueous medium or a gaseous phaseto obtain a manganese oxide, a step of reacting the manganese oxide withan acid in an aqueous medium to substitute proton for a part ofmanganese to obtain a proton-substituted manganese oxide, a step ofreacting the proton-substituted manganese oxide with a lithium compoundin an aqueous medium to obtain a lithium manganate precursor, and a stepof firing the precursor with heating to obtain lithium manganate. Thepresent invention further relates to a positive electrode for lithiumbattery in which the above lithium manganate is used as an activematerial for the positive electrode and to a lithium battery using thepositive electrode.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is an X-ray diffraction chart of sample a.

FIG. 2 is an X-ray diffraction chart of sample A.

FIG. 3 is a scanning electron microphotograph (50,000×magnification)showing the particle structure of sample A.

FIG. 4 is a scanning electron microphotograph (50,000×magnification)showing the particle structure of sample C.

FIG. 5 is a transmission electron microphotograph(1,500,000×magnification) showing the particle structure of sample A.

FIG. 6 is an electron diffraction photograph of sample A.

FIG. 7 is a scanning electron microphotograph (50,000×magnification)showing the particle structure of sample L.

BEST MODE FOR CARRYING OUT THE INVENTION

The present invention relates to a lithium manganate which has a cubicparticle form and contains voids in the particles and provides aninitial discharge capacity of at least 95 mAh/g, preferably at least 100mAh/g when it is used as an active material for positive electrodes oflithium batteries. The lithium manganate may be a single phase or may bea mixture containing lithium manganate and impurities coming from theproduction steps, such as manganese oxide, as far as the above-mentioneddischarge capacity is at least 95 mAh/g. If the discharge capacity islower than the above range, amount of lithium manganate necessary forobtaining batteries of desired capacity increases and this isindustrially not preferred. Lithium manganate in the present inventionis a compound represented by the formula Li_(x)Mn_(y)O₄, and the valuesof x and y in the formula are preferably in the range of 0.3-1.5expressed by the value of x/y. As preferred compositions, mention may bemade of, for example, spinel type LiMn₂O₄ and Li_(4/3)Mn_(5/3)O₄ andlayered rock-salt type LiMnO₂.

The cubic particle form means cubic form such as a die or rectangularparallelopiped form, and the particles include those having angles,namely, apexes or sides which are partially removed. All of theindividual particles are not needed to have the same form, and as far asthey are mainly composed of cubic particles, amorphous particles maypartially be contained.

The presence of voids in the particles can be confirmed by measuringvoid content, and when the void content is 0.005 ml/g or more, it can beadmitted that the particles have voids. The void content is preferably0.01-1.5 ml/g and more preferably 0.01-0.7 ml/g.

Furthermore, that the initial discharge capacity is at least 95 mAh/gwhen the lithium manganate is used as an active material for positiveelectrodes of lithium batteries can be easily confirmed by conductingthe measurement in the state of battery and under the measuringconditions mentioned hereinafter.

By employing the above construction, the lithium secondary batterieswhich contain the lithium manganate of the present invention as anactive material of positive electrodes are high in initial charge anddischarge capacity and excellent in cycle characteristics.

Specific surface area of the lithium manganate is preferably 1-100 m²/g,more preferably 1-30 m²/g. Since this range is preferred for insertionreaction of lithium, when the lithium manganate is used for positiveelectrodes of lithium batteries, no collapse of crystals occurs at thetime of charging and discharging, and battery characteristics areexcellent. The particle diameter is preferably 0.01-10 μm, morepreferably 0.05-5 μm. The particle diameter can be measured by readingthe maximum length of the individual particles in an electronmicrophotograph.

With regard to the process for producing lithium manganate according tothe present invention, the first production process is characterized byincluding {circle around (1)} a step of reacting a manganese compoundwith an alkali to obtain a manganese hydroxide, {circle around (2)} astep of oxidizing the hydroxide in an aqueous medium or a gaseous phaseto obtain a manganese oxide, {circle around (3)} a step of reacting themanganese oxide with a lithium compound in water to obtain a lithiummanganate precursor, and {circle around (4)} a step of firing theprecursor with heating to obtain a lithium manganate. The secondproduction process is characterized by including {circle around (1)} astep of reacting a manganese compound with an alkali to obtain amanganese hydroxide, {circle around (2)} a step of oxidizing thehydroxide in an aqueous medium or a gaseous phase to obtain a manganeseoxide, {circle around (2)}′ a step of reacting the manganese oxide withan acid in an aqueous medium to substitute proton for a part ofmanganese to obtain a proton-substituted manganese oxide, {circle around(3)} a step of reacting the proton-substituted manganese oxide with alithium compound in an aqueous medium to obtain a lithium manganateprecursor, and {circle around (4)} a step of firing the precursor withheating to obtain a lithium manganate.

The step {circle around (1)} is a step of reacting a manganese compoundwith an alkali to obtain a manganese hydroxide. The reaction of amanganese compound with an alkali can be carried out by reacting awater-soluble manganese compound with an alkali in an aqueous medium orby reacting a manganese solution containing Mn²⁺, Mn³⁺ and/or Mn⁴⁺ ionobtained by dissolving a hardly water-soluble manganese compound in anacid with an alkali in an aqueous medium. The former process of reactinga water-soluble manganese compound with an alkali in an aqueous mediumis more preferred. As the water-soluble manganese compounds, there maybe used water-soluble inorganic manganese compounds such as manganesesulfate, manganese chloride and manganese nitrate and water-solubleorganic manganese compounds such as manganese acetate. As the hardlywater-soluble manganese compounds, there may be used MnO₂ and hydratesthereof, Mn₂O₃ and hydrates thereof, manganese oxides such as MnO andMn₃O₄, and organic manganese compounds such as manganese alkoxides. Asthe acids used, mention may be made of inorganic acids such as sulfuricacid, hydrochloric acid and nitric acid, and organic acids such asacetic acid and formic acid. As the alkalis, there may be used alkalihydroxides such as sodium hydroxide, potassium hydroxide and lithiumhydroxide, ammonia compounds such as ammonia gas and aqueous ammonia,and alkali carbonate compounds such as sodium carbonate, potassiumcarbonate, lithium carbonate and ammonium carbonate. The reaction can becarried out in an atmosphere of either air or inert gas, but ispreferably carried out in an inert gas atmosphere for the purpose ofcontrolling the oxidation level of the manganese hydroxide. Reactiontemperature is preferably 10-80° C. for controlling the particle form.

The resulting manganese hydroxide may be filtered or washed, ifnecessary.

The next step {circle around (2)} is a step of oxidizing the manganesehydroxide obtained in the step {circle around (1)} in an aqueous mediumor a gaseous phase to obtain a manganese oxide. The oxidization in anaqueous medium can be carried out by blowing air, oxygen or ozone intoan aqueous medium containing the manganese hydroxide or adding aqueoushydrogen peroxide or a peroxodisulfate to the aqueous medium. Forexample, potassium peroxodisulfate can be used as the peroxodisulfate.Oxidizing temperature in the aqueous medium is preferably 10° C. toboiling point, more preferably room temperature to 90° C. Theoxidization in a gaseous phase can be carried out by filtering orwashing the aqueous medium containing the manganese hydroxide, ifnecessary, and then drying the manganese hydroxide in the air. Oxidizingtemperature in the gaseous phase is preferably room temperature to 300°C., more preferably 50-130° C. In the present invention, it is preferredthat the manganese hydroxide obtained in the step {circle around (1)} is(i) oxidized in an aqueous medium or (ii) first partially oxidized inthe aqueous medium and then oxidized in a gaseous phase. The degree ofoxidation of the manganese hydroxide can be optionally set, but it isconsidered that when the oxidation degree is small, oxides, hydratedoxides or hydroxides of manganese of bivalence, trivalence andtetravalence are present in the manganese oxide. Manganese oxides ofpreferred state in the present invention comprise 2 MnO·MnO₂ as a maincomponent with the molar ratio Mn²⁺/Mn⁴⁺ being in the range of 1-3. Morepreferred are manganese oxides which have a specific surface area of10-40 m²/g, a void content of 0.08-0.3 ml/g, and a particle diameter ofabout 0.08-0.15 μm, and these manganese oxides readily react withlithium compound in the next step {circle around (3)} because of thelarge specific surface area and the high void content. Such manganeseoxides having a large specific surface area and a high void content canbe obtained by employing the above-mentioned preferred oxidizingconditions. In the step {circle around (1)}, while the manganesecompound is reacted with the alkali, for example, while the alkali isadded to an aqueous solution of a manganese compound, the oxidizationmay be carried out with air, oxygen, ozone, aqueous hydrogen peroxide,or peroxodisulfate.

The step {circle around (2)}′ is a step of reacting the manganese oxideobtained in the step {circle around (2)} with an acid in an aqueousmedium to obtain a proton-substituted manganese oxide resulting fromsubstitution of proton for a part of manganese, and theproton-substituted manganese oxide is high in reactivity with thelithium compound in the subsequent step {circle around (3)} of obtaininga lithium manganate precursor, and this is preferred.

As the acid, there may be used any of inorganic acids such ashydrochloric acid, sulfuric acid, nitric acid, hydrofluoric acid and thelike, and water-soluble organic acids such as acetic acid, formic acidand the like. Among them, inorganic acids such as hydrochloric acid,sulfuric acid, nitric acid and hydrofluoric acid are preferred becausethese are industrially advantageous. The temperature at which themanganese oxide is reacted with acid is preferably in the range of roomtemperature to 90° C., more preferably in the range of 40-70° C.

The thus obtained manganese oxide may be filtered, washed or dried, ifnecessary.

The step {circle around (3)} is a step of reacting the manganese oxideor the proton-substituted manganese oxide obtained in the step {circlearound (2)} or {circle around (2)}′ with a lithium compound in anaqueous medium to obtain a lithium manganate precursor. As the lithiumcompound, there may be used lithium hydroxide, lithium carbonate or thelike, and lithium hydroxide is preferred because it is superior inreactivity. The reaction proceeds by mixing the lithium compound and themanganese oxide in an aqueous medium and keeping the temperature at 50°C. or higher. The temperature is more preferably 100° C. or higher,further preferably 100-250° C., and most preferably 100-180° C. When thereaction is carried out at a temperature of 100° C. or higher, it ispreferred to put the lithium compound and the manganese oxide in anautoclave and subject them to hydrothermal treatment under saturatedsteam pressure or under pressurization. Furthermore, when the lithiumcompound and the manganese oxide are mixed in an aqueous medium and themixture is heated, dried and solidified with evaporating the aqueousmedium at 50° C. or higher (evaporation to dryness), concentration ofthe lithium compound in the aqueous medium increases with evaporation ofthe aqueous medium, and, as a result, the lithium manganate precursor isreadily produced upon the reaction of the lithium compound and themanganese oxide, and this is preferred. The lithium manganate precursorobtained in the step {circle around (3)} varies in composition dependingon the reaction conditions, but is considered to be a mixture containingmainly a solid solution of 2MnO·MnO₂ and Li₂O·MnO·MnO₂, LiMn₂O₄, LiMnO₂or the like. This can be confirmed by X-ray diffraction.

When the reaction in the step {circle around (3)} is carried out withfeeding an oxidizing agent by batch-wise or continuous process,reactivity with the lithium compound increases, and this is preferred.The batch-wise process is a process which comprises repeating thefollowing operations (1)-(3) until the reaction reaches the desiredlevel: (1) feeding a given amount of an oxidizing agent to the reactionsystem, then (2) carrying out the reaction with suspending the feedingof the oxidizing agent until the fed oxidizing agent has been consumed,and (3) measuring the reacting weights of the manganese compound and thelithium compound. The batch-wise process is preferred for accuratelycontrolling the reacting weights of the manganese oxide and the lithiumcompound. The continuous process is a process of carrying out thereaction until it reaches the desired reaction level by continuouslyfeeding the oxidizing agent to the reaction system with measuring thereaction level of the manganese oxide and the lithium compound. Thecontinuous process is economically preferred for carrying out thereaction in industrial scale. Moreover, it is preferred to carry out thereaction using at least one oxidizing agent selected from air, oxygen,ozone, aqueous hydrogen peroxide and peroxodisulfate because thereactivity of the lithium compound and the manganese oxide is improved.As the peroxodisulfate, for example, potassium peroxodisulfate can beused.

In order to perform the step {circle around (3)} by hydrothermaltreatment and feeding the oxidizing agent by the batch-wise process,prior to the hydrothermal treatment, air, oxygen or ozone can be blowninto the mixture of the lithium compound and the manganese oxide oraqueous hydrogen peroxide or peroxodisulfate can be added to the mixtureand, further, oxygen can be fed. Furthermore, in the course of thehydrothermal treatment, the temperature is once lowered, and air, oxygenor ozone may be blown into the mixture of the lithium compound and themanganese oxide or aqueous hydrogen peroxide or peroxodisulfate may beadded to the mixture and, further, oxygen may be fed. In the case of thecontinuous process, the hydrothermal treatment is carried out withcontinuously feeding oxygen gas under pressure. The reacting weights ofthe manganese oxide and the lithium compound can be obtained by taking asmall amount of the reaction mixture and measuring by neutralizationtitration the alkali concentration of the solution from which solidmatter has been removed.

The lithium manganate precursor obtained in the step {circle around (3)}may further be oxidized by blowing air, oxygen or ozone into thesolution containing the lithium manganate precursor or adding aqueoushydrogen peroxide or peroxodisulfate to the solution. Furthermore, ifnecessary, the solution may be filtered, washed or dried. The dryingtemperature can be optionally set within the range of lower than thetemperature at which the lithium manganate precursor is converted to alithium manganate, and is suitably 50-200° C.

The step {circle around (4)} is a step of heating and firing the lithiummanganate precursor obtained in the step {circle around (3)} to obtain alithium manganate. The temperature for heating and firing is in therange of from the temperature at which the precursor is converted to thelithium manganate to the temperature at which the specific surface areaof the resulting lithium manganate reaches 1 m²/g or less. It isconsidered that the temperature for heating and firing may varydepending on the composition and particle size of the precursor and thefiring atmosphere, but is generally 250-840° C., and is preferably280-700° C. for obtaining fine lithium manganate of good crystallinity,and more preferably 300-600° C. Furthermore, a range of 650-800° C. ispreferred for obtaining lithium manganate of large particle diameter. Ifthe firing temperature is higher than the above range, lithium in theresulting lithium manganate readily vaporizes. The firing atmosphere isnot limited as far as it is oxygen-containing atmosphere such as air,and the oxygen partial pressure can be optionally set.

Next, the present invention relates to a positive electrode for lithiumbatteries which uses the above-mentioned lithium manganate as an activematerial for the positive electrode, and further relates to a lithiumbattery made using this positive electrode. The lithium batteries in thepresent invention include primary batteries using lithium metal fornegative electrodes, chargeable and dischargeable secondary batteriesusing lithium metal for negative electrodes, and chargeable anddischargeable lithium ion secondary batteries using a carbon material, atin compound, lithium titanate and the like for negative electrodes.

In the case of coin-shaped batteries, the positive electrode for lithiumbatteries can be obtained by adding carbon conductive agents such asacetylene black, carbon and graphite powders and binders such aspolytetrafluoroethylene resin and polyvinylidene fluoride to the lithiummanganate powders of the present invention, kneading them, and moldingthe kneaded product into a pellet. In the case of cylindrical orrectangular batteries, the positive electrode can be obtained by addingthe above additives and, besides, organic solvents such asN-methylpyrrolidone to the lithium manganate powders of the presentinvention, kneading them to prepare a paste, coating the paste on ametallic current collector such as an aluminum foil, and drying thecoat.

As electrolytes of lithium battery, there may be used those which areprepared by dissolving lithium ion in a polar organic solvent which isneither oxidized nor reduced at a potential in a range wider than theelectro-chemically stable range, namely, the potential range in whichthe battery works as a lithium battery. Examples of the polar organicsolvent are propylene carbonate, ethylene carbonate, diethyl carbonate,dimethoxyethane, tetrahydrofuran, γ-butyrolactone, and mixtures thereof.As a solute for lithium ion source, there may be used lithiumperchlorate, lithium hexafluorophosphate, lithium tetrafluoroborate, andthe like. Furthermore, a porous polypropylene film or polyethylene filmis put as a separator between electrodes.

As examples of the battery, mention may be made of coin-shaped batterymade by disposing a separator between a pellet-like positive electrodeand a negative electrode, press-bonding them to a sealed can with apropylene gasket, pouring an electrolyte therein, and sealing it, and acylindrical battery made by coating a positive electrode material and anegative electrode material on metallic current collectors, rolling themwith a separator therebetween, inserting them into a battery can with agasket, pouring an electrolyte therein, and sealing the can. Moreover,there are three-electrode type batteries which are used especially forthe measurement of electrochemical characteristics. These batteries inwhich a reference electrode is disposed in addition to a positiveelectrode and a negative electrode evaluate electrochemicalcharacteristics of the respective electrodes by controlling thepotential of the positive and negative electrodes in respect to thereference electrode.

The performance of the lithium manganate as a positive electrodematerial can be evaluated by constructing a lithium battery in theabove-mentioned manner, charging and discharging the battery at asuitable potential and current, and measuring the electric capacity.Furthermore, cycle characteristics can be judged from the change inelectric capacity caused by repetition of charge and discharge.

EXAMPLE

Examples of the present invention are shown below, but the presentinvention is never limited to these examples.

Example 1

(Synthesis of manganese hydroxide)

815 g of manganese sulfate (86% by weight as MnSO₄) was dissolved inwater to prepare 6.179 liters of a solution. This aqueous manganesesulfate solution was charged in a glass reaction vessel of 10 liters,and, with stirring, 2.321 liters of sodium hydroxide of 4 mols/l inconcentration was added to and dispersed in the aqueous solution over aperiod of 1 hour in a nitrogen atmosphere with keeping the temperatureat 15±5° C. to obtain a manganese hydroxide.

(Synthesis of manganese oxide)

The resulting slurry containing manganese hydroxide was heated to 60°C., and air was blown thereinto for 1 hour to oxidize the manganesehydroxide in an aqueous medium, followed by aging for 1 hour withblowing nitrogen gas in place of the air and thereafter filtering andwashing with water. The resulting filter cake was dried at 110° C. for12 hours to carry out gas phase oxidation to obtain a manganese oxide.This manganese oxide was large in specific surface area and void contentand was mainly composed of 2MnO·MnO₂.

(Synthesis of lithium manganate precursor)

The resulting manganese oxide (240 g in terms of Mn) was dispersed inwater to prepare a slurry. Pure water and 0.920 liter of lithiumhydroxide of 3.206 mols/l in concentration were added to the slurry toobtain 2.40 liters of a liquid. This was charged in a glass reactionvessel of 3 liters and heated to 80° C., and reaction was carried outfor 3 hours with blowing air thereinto. The evaporated water wasreplenished and then a part of the slurry was taken and alkaliconcentration thereof was measured to find that 18.8% by weight of theadded lithium reacted with the manganese oxide. The slurry was chargedin an autoclave and subjected to hydrothermal treatment at 130° C. for 2hours. The slurry was cooled to 80° C. and then alkali concentration inthe slurry was measured in the same manner as above to find that 57.1%by weight of the added lithium reacted with the manganese oxide. Air wasblown into this slurry for 2 hours, and the slurry was again subjectedto hydrothermal treatment at 130° C. for 2 hours to obtain a slurry of alithium manganate precursor (sample a) by batch-wise process. After theslurry was cooled to 80° C., the alkali concentration was measured inthe same manner as above to find that 74.1% by weight of the addedlithium reacted with the manganese oxide. The molar ratio of Li to Mn insample a was 0.50.

An X-ray diffraction chart of the sample a is shown in FIG. 1. From FIG.1, it was recognized that the lithium manganate precursor of sample awas a mixture mainly comprising a solid solution of 2MnO·MnO₂ andLi₂O·MnO·MnO₂, LiMn₂O₄, LiMnO₂ and the like.

(Synthesis of lithium manganate)

Air was blown into the resulting precursor slurry for 2 hours, followedby filtration. Washing was not carried out. The filter cake was dried at110° C., and then fired at 500° C. for 3 hours in the air to obtain alithium manganate (sample A) of the present invention.

Example 2

(Synthesis of manganese hydroxide)

A manganese hydroxide was obtained in the same manner as in Example 1.

(Synthesis of manganese oxide)

A manganese oxide was obtained in the same manner as in Example 1.

(Synthesis of lithium manganate precursor)

The resulting manganese oxide (240 g in terms of Mn) was dispersed inwater to prepare a slurry. Pure water and 0.870 liter of lithiumhydroxide of 3.206 mols/l in concentration were added to the slurry toobtain 2.40 liters of a liquid. This was charged in a glass reactionvessel of 3 liters and heated to 80° C., and reaction was carried outfor 3 hours with blowing air thereinto. The evaporated water wasreplenished and then a part of the slurry was taken, and alkaliconcentration thereof was measured to find that 16.4% by weight of theadded lithium reacted with the manganese oxide. The slurry was chargedin an autoclave and subjected to hydrothermal treatment at 150° C. for 2hours. The slurry was cooled to 80° C. and then alkali concentration inthe slurry was measured in the same manner as above to find that 61.1%by weight of the added lithium reacted with the manganese oxide. Air wasblown into this slurry for 2 hours, and the slurry was again subjectedto hydrothermal treatment at 150° C. for 2 hours to obtain a slurry of alithium manganate precursor (sample b) by batch-wise process. After theslurry was cooled to 80° C., the alkali concentration was measured inthe same manner as above to find that 78.3% by weight of the addedlithium reacted with the manganese oxide. The molar ratio of Li to Mn inthe sample b was 0.50.

(Synthesis of lithium manganate)

Air was blown into the resulting precursor slurry for 2 hours, followedby filtration. Washing was not carried out. The filter cake was dried at110° C., and then fired at 500° C. for 3 hours in the air to obtain alithium manganate (sample B) of the present invention.

Example 3

(Synthesis of manganese hydroxide)

A manganese hydroxide was obtained in the same manner as in Example 1.

(Synthesis of mangane se oxide)

A manganese oxide was obtained in the same manner as in Example 1.

(Synthesis of lithium manganate precursor)

The resulting manganese oxide (186.5 g in terms of Mn) was dispersed inwater to prepare a slurry. Pure water and 0.746 liter of lithiumhydroxide of 3.000 mols/l in concentration were added to the slurry toobtain 2.40 liters of a liquid. This was charged in a glass reactionvessel of 3 liters and heated to 80° C., and reaction was carried outfor 3 hours with blowing air thereinto. The evaporated water wasreplenished and then a part of the slurry was taken, and alkaliconcentration thereof was measured to find that 13.8% by weight of theadded lithium reacted with the manganese oxide. The slurry was chargedin an autoclave and subjected to hydrothermal treatment at 180° C. for 2hours. The slurry was cooled to 80° C. and then alkali concentration inthe slurry was measured in the same manner as above to find that 55.5%by weight of the added lithium reacted with the manganese oxide. Air wasblown into this slurry for 2 hours, and the slurry was again subjectedto hydrothermal treatment at 180° C. for 2 hours to obtain a slurry of alithium manganate precursor (sample c) by batch-wise process. After theslurry was cooled to 80° C., the alkali concentration was measured inthe same manner as above to find that 79.1% by weight of the addedlithium reacted with the manganese oxide. The molar ratio of Li to Mn inthe sample c was 0.52.

(Synthesis of lithium manganate)

Air was blown into the resulting precursor slurry for 2 hours, followedby filtration. Washing was not carried out. The filter cake was dried at110° C., and then fired at 500° C. for 3 hours in the air to obtain alithium manganate (sample C) of the present invention.

Example 4

(Synthesis of manganese hydroxide)

A manganese hydroxide was obtained in the same manner as in Example 1.

(Synthesis of manganese oxide)

The resulting slurry containing manganese hydroxide was heated to 60°C., and air was blown thereinto for 1 hour to oxidize the manganesehydroxide in an aqueous medium, followed by aging for 1 hour withblowing nitrogen gas in place of the air and thereafter filtering andwashing with water. The resulting filter cake was dried at 200° C. for12 hours to carry out gas phase oxidation to obtain a manganese oxide.This manganese oxide was large in specific surface area and void contentand was mainly composed of 2MnO·MnO₂.

(Synthesis of lithium manganate precursor)

The resulting manganese oxide (186.5 g in terms of Mn) was dispersed inwater to prepare a slurry. Pure water and 0.746 liter of lithiumhydroxide of 3.000 mols/l in concentration were added to the slurry toobtain 2.40 liters of a liquid. This was charged in an autoclave andsubjected to hydrothermal treatment at 180° C. for 2 hours. The slurrywas cooled to 80° C. and a part of the slurry was taken and alkaliconcentration thereof was measured to find that 31.9% by weight of theadded lithium reacted with the manganese oxide. Air was blown into thisslurry for 2 hours, and the slurry was again subjected to hydrothermaltreatment at 180° C. for 2 hours. The slurry was cooled to 80° C. andalkali concentration thereof was measured in the same manner as above tofind that 56.6% by weight of the added lithium reacted with themanganese oxide. Air was blown into this slurry for 2 hours, and theslurry was again subjected to hydrothermal treatment at 180° C. for 2hours to obtain a slurry of a lithium manganate precursor (sample d) bybatch-wise process. After the slurry was cooled to 80° C., the alkaliconcentration thereof was measured in the same manner as above to findthat 75.8% by weight of the added lithium reacted with the manganeseoxide. The molar ratio of Li to Mn in sample d was 0.50.

(Synthesis of lithium manganate)

Air was blown into the resulting precursor slurry for 2 hours, followedby filtration. Washing was not carried out. The filter cake was dried at50° C., and a part thereof was fired at 500° C., 700° C. or 800° C. for3 hours in the air to obtain lithium manganates (samples D, E and F) ofthe present invention.

Example 5

(Synthesis of manganese hydroxide)

1146 g of manganese chloride tetrahydrate (99% by weight as MnCl₂·4H₂O)was dissolved in water to prepare 7.153 liters of a solution. Thisaqueous manganese chloride solution was charged in a glass reactionvessel of 10 liters, and, with stirring, 1.847 liters of sodiumhydroxide of 6.209 mols/l in concentration was added to and dispersed inthe aqueous solution over a period of 1 hour in a nitrogen atmospherewith keeping the temperature at 15±5° C. to obtain a manganesehydroxide.

(Synthesis of manganese oxide)

The resulting slurry containing manganese hydroxide was heated to 60°C., and air was blown thereinto for 7 hours to oxidize the manganesehydroxide in an aqueous medium, followed by filtration, washing withwater and re-pulping to obtain a slurry of manganese oxide. Thismanganese oxide had a large specific surface area and a high voidcontent and was mainly composed of 2MnO·MnO₂.

(Synthesis of lithium manganate precursor)

To the resulting manganese oxide slurry (186.5 g in terms of Mn) wereadded pure water and 0.746 liter of lithium hydroxide of 3.000 mols/l inconcentration to obtain 2.40 liters of a liquid. This was charged in aglass reaction vessel of 3 liters and heated to 80° C., and reaction wascarried out for 3 hours with blowing air thereinto. The evaporated waterwas replenished and then a part of the slurry was taken and alkaliconcentration thereof was measured to find that 9.58% by weight of theadded lithium reacted with the manganese oxide. The slurry was chargedin an autoclave and subjected to hydrothermal treatment at 180° C. for 2hours. The slurry was cooled to 80° C. and then a part of the slurry wastaken and alkali concentration of the slurry was measured to find that64.6% by weight of the added lithium reacted with the manganese oxide.Air was blown into this slurry for 2 hours, and the slurry was againsubjected to hydrothermal treatment at 180° C. for 2 hours to obtain aslurry of a lithium manganate precursor (sample g) by batch-wiseprocess. After the slurry was cooled to 80° C., the alkali concentrationwas measured in the same manner as above to find that 75.8% by weight ofthe added lithium reacted with the manganese oxide. The molar ratio ofLi to Mn in sample g was 0.50.

(Synthesis of lithium manganate)

Air was blown into the resulting precursor slurry for 2 hours, followedby filtration. Washing was not carried out. The filter cake was dried at50° C., and then fired at 500° C. for 3 hours in the air to obtain alithium manganate (sample G) of the present invention.

Example 6

(Synthesis of manganese hydroxide)

A manganese hydroxide was obtained in the same manner as in Example 1.

(Synthesis of manganese oxide)

A manganese oxide was obtained in the same manner as in Example 1.

(Synthesis of lithium manganate precursor)

0.304 liter of lithium hydroxide of 3.000 mols/l in concentration wasadded to the resulting manganese oxide (100 g in terms of Mn), followedby thoroughly mixing them with stirring. Then, the mixture wasevaporated to dryness at 110° C. to obtain a lithium manganate precursor(sample h).

(Synthesis of lithium manganate)

The sample h was finely ground using a small-sized grinder and thenfired at 750° C. for 3 hours in the air to obtain a lithium manganate(sample H) of the present invention.

Example 7

(Synthesis of manganese hydroxide)

A manganese hydroxide was obtained in the same manner as in Example 1.

(Synthesis of manganese oxide)

A manganese oxide was obtained in the same manner as in Example 1.

(Synthesis of lithium manganate precursor)

The resulting manganese oxide (324 g in terms of Mn) was dispersed inwater to prepare a slurry. Pure water and 0.966 liter of lithiumhydroxide solution of 3.655 mols/l in concentration were added to theslurry to obtain 2.40 liters of a liquid. This was charged in a glassreaction vessel of 3 liters, and reaction was carried out for 13 hourswith heating to 90° C., and with blowing 1 liter/min of oxygen gasthereinto to obtain a lithium manganate precursor (sample i). Theevaporated water was replenished and then a part of the slurry was takenand alkali concentration thereof was measured to find that 89.4% byweight of the added lithium reacted with the manganese oxide. The molarratio of Li to Mn in the sample i was 0.54.

(Synthesis of lithium manganate)

The resulting precursor slurry was filtered. Washing was not carriedout. The filter cake was dried at 110° C., and then fired at 750° C. for3 hours in the air to obtain a lithium manganate (sample I) of thepresent invention.

Example 8

(Synthesis of manganese hydroxide)

A manganese hydroxide was obtained in the same manner as in Example 1.

(Synthesis of manganese oxide)

The resulting slurry containing manganese hydroxide was heated to 60° C.This slurry had a pH of 8.3. The manganese hydroxide was oxidized in theslurry with blowing 2 liters/min of oxygen gas into the slurry until pHreached 6. Successively, with blowing oxygen gas into the slurry, asodium hydroxide solution of 2 mols/liter in concentration was added toadjust the pH to 9, followed by heating to 90° C., aging for 2 hourswith keeping the pH at 9, filtrating and washing with water. Theresulting filter cake was dispersed in pure water to obtain a slurry of100 g/l in concentration in terms of Mn. This slurry had a pH of 10.7.With stirring, an aqueous hydrochloric acid solution of 1 mol/l inconcentration was added to and dispersed in the slurry at roomtemperature to adjust the pH to 6. With keeping the pH at 6, reactionwas carried out for 3 hours, followed by filtration, washing with waterand drying at 70° C. for 15 hours in the air to obtain a manganeseoxide. This manganese oxide had a large specific surface area and a highvoid content and was mainly composed of 2MnO·MnO₂.

(Synthesis of lithium manganate precursor)

The resulting manganese oxide (312 g in terms of Mn) was dispersed inwater to prepare a slurry. Pure water and 0.877 liter of lithiumhydroxide solution of 3.655 mols/l in concentration were added to theslurry to obtain 2.40 liters of a liquid. This was charged in a glassreaction vessel of 3 liters, and reaction was carried out for 6 hourswith heating to 90° C., and with blowing 1 liter/min of oxygen gasthereinto to obtain a lithium manganate precursor (sample j). Theevaporated water was replenished and then a part of the slurry was takenand alkali concentration thereof was measured to find that 89.8% byweight of the added lithium reacted with the manganese oxide. The molarratio of Li to Mn in the sample j was 0.51.

(Synthesis of lithium manganate)

The resulting precursor slurry was filtered. Washing was not carriedout. The filter cake was dried at 110° C., and then fired at 750° C. for3 hours in the air to obtain a lithium manganate (sample J) of thepresent invention.

Example 9

(Synthesis of manganese hydroxide)

815 g of manganese sulfate (86% by weight as MnSO₄) was dissolved inwater to prepare 6.179 liters of a solution. This aqueous manganesesulfate solution was charged in a glass reaction vessel of 10 liters,and, with stirring, the solution was heated to 60° C. in a nitrogenatmosphere. With keeping 60° C., 2.321 liters of sodium hydroxide of 4mols/l in concentration was added to and dispersed in the aqueoussolution over a period of 1 hour to obtain a manganese hydroxide.

(Synthesis of manganese oxide)

The resulting slurry containing manganese hydroxide had a pH of 8.3.With blowing 2 liters/min of oxygen gas into the slurry, the manganesehydroxide was oxidized in the slurry until pH reached 6, followed byfiltration and washing with water. The resulting filter cake wasdispersed in pure water to prepare a slurry of 100 g/l in terms of Mn,which was charged in a glass reaction vessel of 5 liters and heated to60° C. 1.329 liters of aqueous hydrochloric acid solution of 1 mol/l inconcentration was added to and dispersed in the slurry, and, thereafter,reaction was carried out for 3 hours to replace a part of Mn²⁺ containedin the produced 2MnO·MnO₂ with proton, followed by filtration andwashing with water. By the acid treatment, color of the slurry changedfrom light brown to blackish brown. The slurry after completion of thereaction had a pH of 4.6.

(Synthesis of lithium manganate precursor)

The resulting manganese oxide (312 g in terms of Mn) was dispersed inwater to prepare a slurry. To this slurry were added pure water and0.841 liter of lithium hydroxide solution of 3.655 mols/l inconcentration to obtain 2.40 liters of a liquid. This was charged in aglass reaction vessel of 3 liters, and reaction was carried out for 1hour with heating to 90° C. and with blowing 1 liter/min of oxygen gasthereinto. The evaporated water was replenished and then a part of theslurry was taken and alkali concentration thereof was measured to findthat 55.6% by weight of the added lithium reacted with the manganeseoxide. The slurry was charged in an autoclave and subjected tohydrothermal treatment at 130° C. for 3 hours. The slurry was cooled to90° C. and then alkali concentration of the slurry was measured in thesame manner as above to find that 76.9% by weight of the added lithiumreacted with the manganese oxide. With blowing 1 liter/min of oxygen gasinto this slurry, reaction was carried out at 90° C. for 2 hours toobtain a lithium manganate precursor (sample k). In the same manner asabove, the alkali concentration of the slurry was measured to find that93.7% by weight of the added lithium reacted with the manganese oxide.The molar ratio of Li to Mn in sample k was 0.51.

(Synthesis of lithium manganate)

The resulting precursor slurry was filtered. Washing was not carriedout. The filter cake was dried at 110° C., and then fired at 750° C. for3 hours in the air to obtain a lithium manganate (sample K) of thepresent invention.

Comparative Example 1

(Synthesis of lithium manganate)

50 g of reagent grade manganese dioxide (95% by weight as MnO₂;manufactured by Kanto Kagaku Co., Ltd.) was mixed with lithium hydroxidemonohydrate at a molar ratio of Li/Mn of 0.505. The mixture wasthoroughly mixed and ground by a small-sized grinder, and then chargedin an alumina crucible and fired at 750° C. for 3 hours in the air toobtain a lithium manganate (sample L) of a comparative sample.

Comparative Example 2

(Synthesis of manganese hydroxide)

A manganese hydroxide was obtained in the same manner as in Example 1.

(Synthesis of manganese oxide)

A manganese oxide was obtained in the same manner as in Example 1.

(Synthesis of lithium manganate)

The resulting manganese oxide (50 g in terms of Mn) was mixed withlithium hydroxide monohydrate at a molar ratio of Li/Mn of 0.505. Themixture was thoroughly mixed and ground by a small-sized grinder, andthen charged in an alumina crucible and fired at 750° C. for 3 hours inthe air to obtain a lithium manganate (sample M) of a comparativesample.

Properties of the resulting samples A-M were measured and are shown inTable 1. It was confirmed by observation with an electron microscopethat particle form of the lithium manganates of the present inventionwas cubic. (As examples, scanning electron microphotographs of samples Aand C are shown in FIGS. 3 and 4.) The samples partially containedamorphous particles, but amount of the amorphous particles was slight.On the other hand, the comparative sample M contained partially cubicparticles, but the proportion of amorphous particles in this sample wasvery high as in the comparative sample L. Furthermore, the lithiummanganates of the present invention were recognized to be excellent incrystallinity in view of the results that a spot-like diffractionpattern was obtained in electron diffraction (as an example, an electrondiffraction photograph of sample A is shown in FIG. 6), that a singlelattice image was shown in observation by an ultra-high magnificationtransmission type electron microscope (transmission electronmicrophotograph of sample A is shown in FIG. 5 as an example), and thatthe lithium manganates had a single composition of LiMn₂O₄ as a resultof X-ray diffraction (X-ray diffraction chart of sample A is shown inFIG. 2 as an example).

Furthermore, measurement of specific surface area by BET method and thatof void content by nitrogen adsorption were conducted. BELLSOAP-28manufactured by Japan Bell Co., Ltd. was used for the measurement of thevoid content. The results of measurements are shown in Table 1. It canbe seen from Table 1 that samples A-K had satisfactory specific surfacearea and had voids in the particles.

Next, charge and discharge characteristics and cycle characteristics oflithium secondary batteries using samples A-M as active materials ofpositive electrodes were evaluated. The batteries were oftriple-electrode cells, and charge and discharge were repeated.

The batteries and measuring conditions will be explained.

Each of the samples, a graphite powder as a conductive agent andpolytetrafluoroethylene resin as a binder were mixed at a ratio of3:2:1, and the mixture was kneaded in an agate mortar. The kneadedproduct was molded into circular form of 14 mm in diameter to obtain apellet. Weight of the pellet was 50 mg. This was put between meshes madeof metallic titanium, followed by pressing under a pressure of 150kg/cm² to obtain a positive electrode.

Separately, metallic lithium of 0.5 mm thick was molded into circularform of 14 mm in diameter, and this was put between meshes made ofmetallic nickel, followed by press-bonding to obtain a negativeelectrode. Furthermore, a metallic lithium foil of 0.1 mm thick waswound around a metallic nickel wire so that the foil was in a size of agrain of rice, thereby to obtain a reference electrode. As a non-aqueouselectrolyte, there was used a mixed solution of 1,2-dimethoxyethane andpropylene carbonate (1:1 in volume ratio) in which lithium perchloratewas dissolved at a concentration of 1 mol/l. The electrodes werearranged in the order of positive electrode, reference electrode andnegative electrode, and a porous polypropylene film was put between themas a separator.

The measurement of charge and discharge cycle was carried out at aconstant current setting the voltage range at 4.3 V to 3.5 V and thecharge and discharge current at 0.26 mA (about 1 cycle/day). The initialdischarge capacity, the discharge capacity at 10th cycle and thecapacity retention rate at that time are shown in Table 1. The capacitywas per 1 g of the active material of positive electrode.

TABLE 1 Initial discharge capacity and cycle character- istics ofbatteries using the lithium manganate as active material of positiveelectrode Discharge Specific Initial capacity Capacity surface Voiddischarge at 10th retention Particle Particle area content capacitycycle rate Sample diameter (μm) form (m²/g) (ml/g) (mAh/g) (mAh/g) (%) A0.08 ˜ 0.1  Cubic 20.0 0.140 117.0 114.3 97.7 B 0.08 ˜ 0.2  Cubic 16.50.106 123.5 118.0 95.5 C 0.1 ˜ 0.2 Cubic 10.6 0.069 118.9 114.0 95.9 D0.1 ˜ 0.2 Cubic 10.3 0.085 111.8 107.8 96.4 E 0.1 ˜ 0.6 Cubic + 4.10.018 124.0 113.8 91.8 Amorphous F 0.5 ˜ 0.6 Cubic + 1.7 0.010 123.0110.6 90.0 Amorphous G 0.1 ˜ 0.2 Cubic 12.3 0.083 120.3 114.8 95.4 H 0.1 ˜ 0.25 Cubic + 3.9 0.025 126.6 120.7 95.3 Amorphous I 0.2 ˜ 0.6Cubic + 3.6 0.010 117.1 115.2 98.4 Amorphous J 0.2 ˜ 0.4 Cubic 6.0 0.014130.6 126.8 96.8 K 0.4 ˜ 0.5 Cubic 3.5 0.010 130.2 124.6 95.7 L 0.1˜Amorphous 1.5 0.007 91.4 87.2 95.4 M 0.2 ˜ 0.4 Cubic + 2.8 0.006 68.266.0 96.9 Amorphous

As shown in Table 1, the lithium manganates of the present inventionshowed a high initial discharge capacity of at least 95 mAh/g and,besides, provided excellent cycle characteristics.

Lithium manganate prepared by the conventional dry process was apt tohave defects in crystal structure, which cause deterioration ofcrystallinity with repetition of charge and discharge, resulting indecrease of cycle capacity.

Moreover, lithium manganate is smaller in diffusion coefficient oflithium ion as compared with lithium cobaltate having a layered rocksalt structure which has been practically used as active material forpositive electrode of lithium ion secondary batteries. On the otherhand, the lithium manganate of the present invention has cubic particleform and contains voids in the particles, and thus is excellent incrystallinity. That is, it has conditions advantageous for insertion oflithium, and, hence, is preferred for attaining improvement of currentdensity.

INDUSTRIAL APPLICABILITY

The lithium manganate of the present invention has a cubic particle formand contains voids in the particles, and, therefore, lithium batteriesusing it as a positive electrode material show a high initial dischargecapacity and furthermore are excellent in cycle characteristics. Inaddition, the production process of the present invention is a processaccording to which lithium manganates having the above features can beadvantageously produced.

What is claimed is:
 1. A process for producing a lithium manganate whichincludes a step of reacting a manganese compound with an alkali toobtain a manganese hydroxide, a step of oxidizing the manganesehydroxide in an aqueous medium or a gaseous phase to obtain a manganeseoxide, a step of reacting the manganese oxide with a lithium compound inan aqueous medium to obtain a lithium manganate precursor, and a step offiring the precursor to obtain a lithium manganate.
 2. A processaccording to claim 1, wherein the manganese oxide and the lithiumcompound are subjected to hydro-thermal treatment in an aqueous mediumin the step of reacting the manganese oxide with a lithium compound inan aqueous medium to obtain a lithium manganate precursor.
 3. A processaccording to claim 1, wherein an oxidizing agent is fed into a reactorin batch-wise or continuous manner in the step of reacting the manganeseoxide with a lithium compound in an aqueous medium to obtain a lithiummanganate precursor.
 4. A process according to claim 1 wherein thereaction of manganese oxide with a lithium compound in an aqueous mediumto obtain a lithium manganate precursor occurs at a temperature of 50°C. or higher.
 5. A process for producing a lithium manganate whichincludes a step of reacting a manganese compound with an alkali toobtain a manganese hydroxide, a step of oxidizing the manganesehydroxide in an aqueous medium or a gaseous phase to obtain a manganeseoxide, a step of reacting the manganese oxide with an acid in an aqueousmedium to obtain a proton-substituted manganese oxide, a step ofreacting the proton-substituted manganese oxide with a lithium compoundin an aqueous medium to obtain a lithium manganate precursor, and a stepof firing the precursor to obtain a lithium manganate.
 6. A processaccording to claim 5, wherein the acid is at least one acid selectedfrom hydrochloric acid, sulfuric acid, nitric acid and hydrofluoricacid.
 7. A process according to claim 1 or 5, wherein the manganesecompound is a water-soluble manganese compound.
 8. A process accordingto claim 5, wherein the manganese oxide or the proton-substitutedmanganese oxide and the lithium compound are subjected to hydro-thermaltreatment in an aqueous medium in the step of reacting theproton-substituted manganese oxide with a lithium compound in an aqueousmedium to obtain a lithium manganate precursor.
 9. A process accordingto claim 1 or 5, wherein the lithium compound is lithium hydroxide. 10.A process according to claim 5, wherein an oxidizing agent is fed into areactor in batch-wise or continuous manner in the step of reacting theproton-substituted manganese oxide with a lithium compound in an aqueousmedium to obtain a lithium manganate precursor.
 11. A process accordingto claim 10, wherein the oxidizing agent is at least one agent selectedfrom air, oxygen, ozone, aqueous hydrogen peroxide and peroxodisulfate.12. A process according to claim 5 wherein the reaction of theproton-substituted manganese oxide with a lithium compound in an aqueousmedium to obtain a lithium manganate precursor occurs at a temperatureof 50° C. or higher.
 13. A process according to claim 1 or claim 5,wherein the produced lithium manganate is employed in the production ofa positive electrode.
 14. A process according to claim 1 or claim 5,wherein the produced lithium manganate is employed in the production ofa lithium battery.